This invention relates generally to waveguide transmission lines and more particularly to waveguide transition circuits.
As is known in the art, a waveguide transmission line is a structure used to guide electromagnetic waves generally referred to as radio frequency (RF) signals. Waveguide transmission lines provide a transmission media having a relatively low insertion loss characteristic to RF signals fed thereto. Thus, waveguide transmission lines are used in those RF receiving and transmitting systems requiring RF transmission lines having a relatively low insertion loss characteristic.
As is also known in the art, a radar system is one particular type of RF system which detects RF signals. The radar system generally includes an antenna, a transmitter and a receiver. The radar transmitter emits an RF signal from the antenna. Portions of the transmitted RF signal are intercepted by reflecting objects (e.g. a target) and reflected back to the radar. The reflected RF signal is generally referred to as an echo signal. The antenna receives the echo signal and feeds the echo signal to the receiver.
Noise is unwanted RF energy which interferes with the ability of the receiver to detect the echo signal. The capability of the receiver to detect a weak echo signal is limited by the noise energy that occupies the same portion of the frequency spectrum as does the echo signal.
The echo signal and noise energy from external sources may both enter the radar receiver via the antenna. Noise energy may also originate in the receiver itself due to various causes such as thermal motion of the conduction electrons in ohmic portions of those circuit components which are disposed to provide the receiver.
The receiver cannot detect any echo signal having a power level below the power level of the noise energy. The weakest signal the receiver may detect is generally referred to as the minimum detectable signal. In the absence of a so-called jamming signal, the noise energy of external sources is generally relatively low. Thus the power level of the minimum detectable signal is limited by the noise of the radar receiver.
The effectiveness of the receiver to detect echo signals in the presence of noise energy may be represented by a figure of merit generally referred to as the noise figure of the receiver. In general, the noise figure of a radar receiver may be defined as the input signal to noise ratio divided by the output signal to the noise ratio. The input signal to noise ratio is provided by the ratio of the input signal power to the input noise power. Similarly, the output signal to noise ratio is provided by the ratio of the receiver output signal power to the output noise power. Thus, the noise figure may be interpreted as a degradation of the input signal to noise ratio as the echo signal passes through the receiver.
In the ideal case the receiver should not degrade the input signal to noise ratio. Therefore, in the ideal case, the noise figure of the receiver is unity (i.e. 0 decibels).
Every circuit component disposed between the antenna terminals and the intermediate frequency (IF) port of the receiver contributes to the noise figure of the receiver. A radar system having a so-called low noise receiver generally includes a low noise amplifier (LNA) and a mixer. The LNA is provided having a high gain characteristic and a noise figure close to unity. The contribution to the overall receiver noise figure from those components which follow the LNA is reduced by the gain characteristic of the LNA. Thus, to provide a receiver having a relatively low noise figure the LNA is generally provided as the first active component of the receiver.
The noise figure of a passive circuit component such as an RF transmission line for example, corresponds to the insertion loss characteristic of the component. RF transmission lines generally couple the antenna to the LNA. In a radar system provided with a low noise amplifier, the echo signal should ideally be coupled from the antenna to the LNA with no attenuation and therefore no increase in the noise figure of the receiver.
In practical radar systems, however, RF transmission lines which couple the antenna to the LNA may attenuate the echo signal and thus provide the receiver with an increased noise figure. Therefore, it is desirable to couple the echo signal from the antenna to the LNA with a minimum insertion loss characteristic.
The output ports of an RF antenna are often provided as waveguide transmission lines. The input and output ports of RF circuit components such as low noise amplifiers, mixers and the like, however, are generally provided as coaxial or strip conductor transmission lines. This is particularly true in those RF systems which operate in the microwave and millimeter wave frequency ranges. Thus, it is necessary to provide a transition between the waveguide transmission line of the antenna, for example, and the coaxial, or strip conductor transmission line of an RF circuit component such as an LNA for example.
Furthermore, a typical waveguide transmission line has a dominant mode wave impedance characteristic typically of about 377 ohms (.OMEGA.). However, coaxial and strip conductor transmission lines are generally provided having an impedance characteristic typically of about 50 .OMEGA.. Thus the transition means should provide both a physical transition and an electrical impedance transition between the waveguide transmission line and the coaxial or the strip conductor transmission line.
Moreover in radar systems having a low noise receiver, the transition should not degrade the noise figure of the low noise receiver. Thus, the transition should have a relatively low insertion loss characteristic.
A so-called waveguide transition circuit is used to transition from a waveguide transmission line to a coaxial or strip conductor transmission line. One type of waveguide transition circuit used to transition between waveguide and microstrip transmission lines is a so-called ridge-waveguide transition circuit. In the ridge-waveguide transition circuit, the waveguide is provided with a series of steps or ridges along the longitudinal axis of the waveguide transmission line. The lengths of the ridges are selected to correspond to an electrical pathlength typically of about one quarter wavelength at a predetermined frequency. Each successive ridge reduces the height of the waveguide transmission line.
A reduction in the height of a waveguide transmission line provides a change in the impedance characteristic of the waveguide transmission line. This change in impedance characteristic is similar to the change in impedance characteristic provided by a so-called quarter wave transformer circuit well known in the art and generally used in coaxial or strip conductor transmission lines. Thus, in the ridge waveguide transition circuit, the waveguide height is reduced until the impedance characteristic of the waveguide transmission line corresponds to the impedance characteristic of the microstrip transmission line.
Several problems exist with the ridge waveguide transition approach. First, because each ridge is one-quarter wavelength long, the resultant size of practical ridge waveguide transition circuits is relatively large. Second, at microwave and millimeter wave frequencies it is relatively difficult to provide the ridges in the waveguide with the required tolerances. Third, it is relatively difficult to provide a ridge waveguide transition circuit as a hermetically sealed circuit.
A second type of waveguide transition circuit is the so-called end launch transition circuit described in a paper titled "Coaxial-to-Waveguide Transition (End-Launcher Type)" by B. N. Das and G. S. Sanya published in the Proceedings of the IEE Vol. 123 No. 10 October 1976.
This paper describes a circuit which provides a transition between a coaxial transmission line and a waveguide transmission line. The outer conductor and dielectric portion of the coaxial transmission line are stripped away from the center conductor of the coaxial line. The center conductor of the coaxial line is then inserted into the waveguide. A conductive tab is connected between the center conductor of the coaxial line and an internal wall of the waveguide. The connection between the center conductor, the conductive tab and the internal wall of the waveguide provides a conductive loop which couples magnetic fields in the waveguide as is generally known. The coaxial electromagnetic mode is thus transformed to the dominant waveguide mode.
Several problems exist with this approach. First, the conductive tab which connects the center conductor of the coaxial line to the internal wall of the waveguide is soldered to both the coaxial line and the waveguide wall. At microwave and millimeter wave frequencies, the waveguide dimensions become relatively small and thus it becomes difficult to reliably provide such solder connections.
Second, this type of transition circuit is sensitive to variations in construction and assembly processes. Variations in the assembly process cause end launch transition circuits to have a relatively high insertion loss characteristic at the desired frequency of operation.
Third, this type of transition is not hermetically sealed. In many applications such as missile guidance radars, variations in environmental conditions such as temperature, humidity and altitude may result in condensed water being disposed in the waveguide. A waveguide having water disposed therein has an increased insertion loss characteristic, a degraded impedance characteristic and a reduced power handling capability. Thus, it is desirable to provide a waveguide transition circuit as a hermetically sealed waveguide transition circuit.